(A) Field of the Invention
The present invention relates to an over-current protection device and a manufacturing method thereof, and more particularly, to an SMD (surface mount device) over-current protection device with a positive temperature coefficient (PTC) characteristic.
(B) Description of the Related Art
The resistance of a positive temperature coefficient conductive material is sensitive to temperature variation and can be kept extremely low during normal operation so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g., above 104 ohm.) Therefore, the over-current will be eliminated and the objective to protect the circuit device will be achieved. Consequently, PTC devices have been commonly integrated into various circuitries so as to prevent damage caused by over-current events.
FIG. 1 shows an over-current protection electrical apparatus 1 disclosed by U.S. Pat. No. 5,852,397. Metallic foil electrodes 11 are respectively attached to the upper surface and lower surface of a PTC material layer 10. Next, the surfaces of the metallic foil electrodes 11 are etched to form long grooves 12 so that each of the metallic foil electrodes 11 is divided into two electrode portions of different sizes. Through holes are drilled on the left and right edges of the electrical apparatus 1, and each of the through holes is respectively filled with a conductive rod 13 by plating. Therefore, all the electrode portions on a side are electrically connected to each other along a vertical direction.
FIG. 2 is a perspective diagram of an SMD electrical apparatus disclosed by R.O.C U.S. Pat. No. 415,624. The SMD electrical apparatus 2 has a PTC material layer 10, and metallic foil layers 21 are respectively attached to the upper surface and lower surface of the PTC material layer 10. Next, long slots are respectively formed on the left edge of the upper metallic foil layer 21 and the right edge of the lower metallic foil layer 21 by etching. Insulating films 22 are respectively coated on the upper and lower metallic foil layers 21, and the slots are also filled with the insulating films 22. Subsequently, metallic foil electrodes 23 are respectively attached to the upper and lower insulating films 22. Similarly, an etching process is used to remove the middle portions of the metallic foil electrodes 23, and symmetric left and right portions are left on two sides.
Through holes are formed on the left and right edges of the SMD electrical apparatus 2. Conductive layers 24 are plated on the surfaces of the through holes so that the two left metallic foil electrodes 23 are connected to the lower metallic foil layer 21 and two right metallic foil electrodes 23 are connected to the upper metallic foil layer 21.
The aforesaid prior arts all utilize methods similar to the manufacturing processes of printed circuit board, such as exposure, development, etching, drilling and plating. Therefore, not only do the prior arts require expensive equipment and complicated processes to manufacture, but also produce harmful etching liquids or plating liquids that pollute the environment.
In addition, regarding the electrical apparatus 1 and the SMD electrical apparatus 2, the external electrodes and internal electrodes (or conductive rods and conductive layers) have smaller contact areas, so the electrical resistance is raised. Considering the ever-progressing requirements for reducing the size of electrical devices, the contact areas or the diameters of the through holes cannot be effectively reduced due to their inherent limitations. The prior arts are therefore not suitable for manufacturing miniature over-current protection devices.
Also, in the prior art, the two sides of the electrical apparatus 1 (the surface perpendicular to the upper and lower metallic foil layers, and the surface along the lengthwise direction of the main body) and the SMD electrical apparatus 2 have stacked layers exposed to the atmosphere. Consequently, moisture penetrates the PTC material layer and the upper and lower metallic foil layers so that the electrical reliability is affected.